Never underestimate the power of a cheap, well-marketed tablet like Amazon's Kindle Fire. According to Comscore, the Kindle Fire accounts for 54.4 percent of Android tablets sold in the United States.

No other Android tablet comes close in Comscore's figures. Samsung's entire Galaxy Tab lineup only has 15.4 percent of the Android market. Motorola's Xoom -- the first Android tablet on the market -- has only a 7 percent share.

But the numbers are skewed for one big reason: Barnes & Noble's Nook Tablet and Nook Color are omitted from the measurements. Comscore considers those Nooks to be e-readers, not tablets -- even though they run apps, browse the Web, and play music and video on full color displays, just like the Kindle Fire.

Barnes & Noble would likely capture a sizable share if its slate was included. Before the Kindle Fire launched last year, the Nook Color was the second best-selling tablet on the market, behind Apple's iPad.

In any case, Comscore's numbers confirm what most tech watchers already assumed: Android tablets owe most of their success to the Kindle Fire and presumably the Nook tablets, which use heavily modified versions of open-source Android. Their interfaces are designed around simple media consumption, and their pricing starts at just $200, making them safe, low-budget alternatives to the iPad. A study from January found that Android's tablet share tripled to 39 percent in the fourth quarter, when Amazon and Barnes & Noble launched their tablets.

The success of the Kindle Fire and Nook tablets likely vexes Google, because its own app store and content services aren't available on those devices. Google is now trying to bolster its own ecosystem by re-branding it as “Google Play,” and will likely launch its own low-cost, 7-inch tablet this year.

That may be Google's best hope at gaining ground, because no Android tablet that's taking on the iPad directly is getting any traction.